Development of Atomic Structure

How long have people been interested in understanding matter and its structure?

A. Thousands of years

B. Hundreds of years

C.A few years

D.Never

• Answer: A

Democritus (ca. 450 BC)

Ancient Philosophy

• Who: Aristotle & Democritus• When: More than 2000 years ago• Where: Greece• What: Aristotle believed in 4 elements: Earth,

Air, Fire, and Water. Democritus believed that matter was made of small particles he named “atoms”.

• Why: Aristotle and Democritus used observation and inference to explain the existence of everything.

Alchemists

• Who: European Scientists

• When: 800 – 900 years ago

• Where: Europe

• What: Their work developed into what is now modern chemistry.

• Why: Trying to change ordinary materials into gold.

Alchemic Symbols

John Dalton (1766-1844)

• Based his work on Lavoisier (Law of Convservation of Mass)

Dalton’s Atomic TheoryAtomic Theory (1830)

1) All matter is made of atoms. Atoms are indivisible and indestructible.

2) All atoms of a given element are identical in mass and properties.

3) Compounds are formed by a combination of two or more different kinds of whole atoms.

4) A chemical reaction is a rearrangement of atoms.

J.J. Thomson (1856-1940)

• Discovery of ELECTRON

Thomson’s Cathode Ray Cathode Ray ExperimentExperiment (1897)

• Ray of electrically charged particles through vacuum tube

• Ray “bent” towards positive plate• Must be some negative component to matter

Ernest Rutherford (1871-1937)

• Discovery of NUCLEUS

Rutherford’s Gold Foil ExperimentGold Foil Experiment (1909)

• Shot alpha particles (+) through gold foil

• 98% passed through undeflected

• 2% deflected back

Therefore…• Small, dense,

positively charged mass in center of atom deflecting 2% of alpha particles

• Called NUCLEUS (contains protons and neutrons)

• Atom is mostly EMPTY SPACE

Subatomic ParticlesSubatomic Particles

Particle Symbol Location Charge Mass

Electron e-

Surrounding nucleus -1 0

Proton p+

Inside nucleus +1 1

Neutron n0

Inside nucleus 0 1